1. Field of the Invention
This invention relates generally to diode pumped lasers, and more particularly to a diode pumped laser with high efficiency that is achieved by tailoring an intensity profile to the pump beam rather than the laser cavity.
2. Description of Related Art
Today, mode matching is well understood. In mode matching, the laser cavity has a fundamental mode (TEMoo) of a given diameter. The laser radiation from a pump source, including but not limited to a diode or Ti: Sapphire laser, is focused into this volume in a spot diameter smaller than the diameter of the TEMoo mode. Provided the TEMoo mode is larger than the pump spot, the laser can only laze TEMoo. Regardless of where the pump spot is placed within the TEMoo mode diameter, the laser produces a consistent TEMoo mode.
In the ideal mode-matching case, the pump spot has a gaussian intensity profile and matches perfectly to the gaussian profile of the TEMoo cavity mode. Researchers have struggled to improve the beam quality of laser diodes so their outputs can be coupled into smaller spots with lower NA--i.e, to make them more a like the Ti:sapphire lasers employed in the early days of end pumping. The recent surge in porgies with fiber lasers has stemmed almost entirely from the improvements in diode "brightness" allowing 10's of Watts of diode light to be coupled into 200 .mu.m spots with low NA.
In a conventional side-pumped laser rod using flash pumps, the pump energy is deposited uniformly across the diameter of the laser rod, leading to a parabolic temperature distribution which causes a conventional spherical thermal lens. Spherical lenses can be accounted for in resonator design and also corrected using intracavity spherical lenses of opposite magnitude. In side-pumped lasers using diodes, the diode light is focused into the laser rod (especially in lower gain CW pumped systems). Such focusing maximizes gain and makes advantage of the optical quality of the diode pump light and its ability to overlap directed energy beam to a common spot in the center of the laser rod. The act of focusing induces strong aberrations in the thermal lens in such systems.
The act of end-pumping the laser places an intense heat source in the center of a crystal and causes an aberration in the parabolic temperature distribution and this aberration makes an Aspheric lens. The Aspheric lens cannot be compensated using spherical optics, and the cavity modes which have spherical wavefronts are distorted by the Aspheric lens. The result is that the cavity mode is changed in a way that cannot be predicted using the traditional laser designer's tools of ABCD matrixes which allow calculation of round-trip parameters and mode sizes using gaussian beam optics. The result is that the ensuing laser output is not TEMoo even when the pump spot is smaller that the TEMoo mode volume calculated using conventional means. Furthermore, even if the pump spot is made much smaller that the TEMoo mode size calculated by ABCD matrices, the output is far from TEMoo.
Researchers have corrected this phenomena by employing intracavity correcting optics that are fabricated with the Aspheric lens surface induced by end-pumping at the expense of additional complexity and losses caused by the imperfect surfaces of diamond turned optics. Additionally, this correction is perfect at only one pump power since the lens shape changes with pump power.
Researches have limited the lazing mode to the central region of the pump volume where the lensing is more spherical and employs special confocal or concentric laser cavities. This allows for expansion of the cavity mode in the laser crystal under strong thermal focusing. In general it is well understood that placing a strong lensing laser crystal in the center of a nearconfocal or concentric resonator will cause the TEMoo mode to expand within the crystal making mode matching between pump and TEMoo mode easier.
As pump power is scaled to higher levels, the central pump region over which lensing is spherical becomes smaller while the Aspheric region in the edges of the pump region expands. In order to extract TEMoo output the cavity mode must be constrained to this successively smaller region with a resulting reduction in efficiency.
These current methods of confining the TEMoo cavity mode to the central pump region, where the OPD is parabolic & lensing spherical, results in a dramatic reduction in extraction efficiency. This is particularly true for lasers pumped with low to moderate pump powers of 10w or less. However, at higher pump powers the thermal lensing produced in most gain media, such as Nd:YAG, or Nd:YV04, becomes significant.
There is a need for a diode pumped laser which is efficient at high pump powers equal to or exceeding 10 W. There is a further need for a diode pumped laser with high TEMoo mode extraction efficiency. Yet a further need exists for a diode pumped laser where the intensity profile is tailored to the pump beam rather than the laser cavity. Another need exists for a diode pumped laser where the gain media is pumped by a beam with a quasi-top-hat pump profile. Still another need exists for a diode pumped laser where the gain media is pumped by a beam with a sharp decrease in pump intensity at the edges of the pump region.